In 3GPP LTE, OFDMA (Orthogonal Frequency Division Multiple Access) is employed as a downlink communication method. In a radio communication system adopting 3GPP LTE, a base station transmits a synchronizing signal (synchronization channel: SCH) and a broadcast signal (broadcast channel: BCH) using prescribed communication resources. A terminal first synchronizes with a base station by capturing the SCH. Then, the terminal acquires parameters that are specific to that base station (for example, a frequency bandwidth) by reading BCH information (see Non-patent Literature 1, 2 and 3).
Also, after the terminal acquires base station-specific parameters, the terminal sends a connection request to the base station, and, by this means, establishes communication with the base station. When necessary, the base station transmits control information to the terminal, with which communication has been established, using a PDCCH (Physical Downlink Control CHannel).
The terminal performs “blind detection” for a received PDCCH signal. That is, a PDCCH signal includes a CRC (Cyclic Redundancy Check) part, and, at a base station, this CRC part is masked by the terminal ID of the target terminal. Thus, until a terminal demasks the CRC part of a received PDCCH signal with the terminal's terminal ID, the terminal cannot decide whether or not the PDCCH signal is for that terminal. In this blind detection, if the result of demasking is that CRC calculation is OK, the PDCCH signal is decided to have been sent for the terminal.
Also, control information sent from a base station includes assignment control information including information about resources which a base station allocates to a terminal. A terminal needs to receive both downlink assignment control information and uplink assignment control information which have a plurality of formats. Although downlink assignment control information which a terminal should receive can be defined in a plurality of sizes depending on the transmitting antenna control method and frequency allocation method at a base station, and a terminal identifies the format using this size difference, some of these downlink assignment control information formats (hereinafter simply referred to as “downlink assignment control information”) and uplink assignment control information formats (hereinafter simply referred to as “uplink assignment control information”) are transmitted in a PDCCH signal which has the same size. Downlink assignment control information and uplink assignment control information which have this same information size include type information of assignment control information (for example, a 1-bit flag). Thus, even if the size of a PDCCH signal including downlink assignment control information and the size of a PDCCH signal including uplink assignment control information are the same, a terminal checks type information of assignment control information, and by this means can distinguish between downlink assignment control information and uplink assignment control information. The PDCCH format to transmit uplink assignment control information is PDCCH format 0, and the PDCCH format to transmit downlink assignment control information, transmitted in a PDCCH signal being the same size as uplink assignment control information, is PDCCH format 1A.
However, cases might occur where the information size of uplink assignment control information determined from the uplink bandwidth (that is, the number of bits required for transmission) and the information size of downlink assignment control information determined from the downlink bandwidth differ. To be more specific, if an uplink bandwidth is small, the information size of uplink assignment control information becomes small, and, if a downlink bandwidth is small, the information size of downlink assignment control information becomes small. If a difference of bandwidth results in a difference of information size like this, by adding zero information to the smaller assignment control information (that is, by performing zero-padding), the size of downlink assignment control information and the size of uplink assignment control information are made equal. By this means, whether the content is downlink assignment control information or uplink assignment control information, PDCCH signals have the same size. The size adjustment of control information as mentioned above reduces the number of times of blind detection at a terminal on the receiving side.
Also, the standardization of 3GPP LTE-advanced has been started to realize much faster communication than 3GPP LTE. 3GPP LTE-advanced system (hereinafter referred to as “LTE-A system”) follows 3GPP LTE system (hereinafter referred to as “LTE system”). In 3GPP LTE-advanced, to realize an uplink transmission speed of maximum 500 Mbps or greater and improve uplink frequency use efficiency, MIMO (Multi-Input Multi-Output) in uplink communication is expected to be introduced. Thus, a terminal has multiple transmitting antennas, and controls an uplink transmission weight (that is, precoding vector) and the number of uplink channel data subject to spatial multiplex (that is, the number of spatial layers) based on the command of a base station.
Also, to improve uplink frequency use efficiency, studies are underway to allocate uplink data in the frequency domain discontinuously (hereinafter simply referred to as “discontinuous allocation”). In this case, for example, OFDM and clustered DFT-s-OFDMA are applied (see Non-patent Literature 4). Thus, while conventional LTE only supports a continuous allocation in a frequency domain because of a limitation of SC-TDMA, it is possible in an LTE-A system to allocate a high quality subcarrier adaptively to a terminal in the frequency domain, so that uplink frequency use efficiency is expected to be improved.